Lecturer, Graduate School of Engineering Science, Osaka University
We have developed techniques for measuring electrical resistance and magnetization at ultra-high pressure and at very low temperature. A diamond-anvil cell (DAC) is popular and the most powerful tool for producing static pressure. We have constructed a compact non-magnetic DAC specially designed for low temperature research. We recently attained to perform electrical measurements above 200 GPa at temperature down to 30 mK by assembling on a powerful 3He/4He dilution refrigerator. The combination of two powerful tools has made it possible to study the condensed matter physics in unprecedentedly wide pressure-temperature range.
One of the most aggressive challenges under high pressure research is the search for metallic hydrogen under its extremely dense phase. The metallic hydrogen is predicted to show superconductivity even at room temperature and has long fascinated the high-pressure physicists, which exists in fact at the interior of a giant planet such as Jupiter or Saturn. Another challenge topic is “superconductivity” and “magnetism”. It is well known that a ferromagnetic metal does not show superconductivity and even a small amount of paramagnetic impurities could suppress the superconductivity. However, even in the case of iron, the superconducting transition temperature (Tc) was theoretically predicted to be 0.25 K. We can expect an appearance of superconductivity in magnetic metals in its nonmagnetic state under certain pressure and low temperature.
Electrical transport measurements in DAC can provide both the proof of metallization and superconductivity at low temperature. Conductivity measurements are difficult in general at Mbar-pressures using DAC because of the wiring problem to the small sample and insulation between the wire and the metal gasket. We succeeded to perform a four-probe electrical resistance measurement in DAC at pressure of 250 GPa in non-hydrostatic measurements; 10 GPa in hydrostatic ones using a ‘liquid’ pressure medium where a piston-cylinder type hydrostatic cell delimitates the maximum pressure of around 3 or 4 GPa.
Using these experimental technique, we have studied searching for metallization and superconductivity in simple systems such as elements like halogen atoms, chalcogen atoms, 3d-magnetic metals, and alkaline metals. Recently, the high-pressure research of superconductivity in heavy-fermion systems is a popular area. We also apply these technical advances to the area where hydrostatic pressure is required since axial stress causes the dislocation which is considered to suppress the superconductivity.
Fig.1) Coherent oscillations observed in the first Cooper pair box experiment.
As a next step, we decided to do a time-domain experiment to demonstrate quantum state control of the charge-number superposition. That was also the time when ideas of quantum information processing and quantum computing started to spread around the community. We needed to develop a few new experimental techniques, such as the ultrafast gate-voltage pulses for the control and the time-ensemble current measurement for the readout, but finally succeeded to observe coherent oscillations (Figure 1) between charge number states, in autumn 1998. The paper was published in Nature, in April 1999. It was well accepted and triggered research on solid-state quantum computing.